Study on the effect of interface failure between casing and cement sheath on casing stress under non-uniform in-situ stress

Abstract The interface failure (micro-annulus and de-bonding) at the first interface between cement sheath and formation, or at the second interface between cement sheath and casing can be easily caused by alternating change of internal pressure and temperature in the casing during large-scale multistage hydraulic fracturing in shale-gas well. It is likely to have a great impact on casing stress and pose a potential threat to casing failure, especially the casing under non-uniform in-situ stress (NUS). Hence, based on the generation mechanism of micro-annulus in the first and second interface, the mechanical model coupling the casing and cement sheath as well as formation under non-uniform in-situ stress is presented with due consideration of the effect of interface failure status on casing stress. In the model, the failure interface with micro-annulus is considered by adopting boundary conditions of smooth contact, and the interface without micro-annulus is considered by adopting boundary conditions of ideal complete contact. The stress and displacement equations are derived by the semi analytical method, and the linear algebra equations and analytical solution of the equations are obtained based on the different contact status. The associated calculating program to calculate stress distribution of casing and cement sheath is developed and programmed. The effects of physical parameter and geometrical parameters of formation, cement sheath and casing, contact status of cement sheath boundary and non-uniformity of in-situ stress on casing stress have been systematically investigated.